Systems for measuring the intra-arterial blood pressure of a patient can be subdivided into two main groups--those which invade the arterial wall to access blood pressure and those which use noninvasive techniques. Initially, the most accurate blood pressure measurements were achievable only by use of invasive methods. One such common method involved use of a fluid filled catheter inserted into a patient's artery. While invasive methods provide for accurate blood pressure measurements, the risks of infection and other complications, in many cases, outweigh the advantages of using invasive techniques.
Because of the above-mentioned risks associated with invasive methods, a noninvasive method, known as the Korotkoff method is widely used. The Korotkoff method is known as an auscultatory method because it uses, in part, the characteristic sound made as the blood flows through the artery to aid in measuring blood pressure. Although the Korotkoff method is noninvasive, it only provides a measurement of the highest pressure point (systolic) and the lowest pressure point (diastolic) along the pressure wave. While, for many purposes, systolic and diastolic pressure are sufficient, there are many applications in which it is desirable to use the entire characteristic curve of the blood pressure wave. In these applications, the Korotkoff method simply is incapable of providing satisfactory information. In addition to this limitation of the Korotkoff method, it necessitates the temporary occlusion of the artery in which blood pressure is being monitored. While arterial occlusion is not prohibitive in many applications, there are occasions where the patient's blood pressure must be monitored continuously (such as when undergoing surgery) and accordingly, the prohibiting of blood flow, even on a temporary basis, is undesirable.
Because of the above-mentioned risks associated with invasive blood pressure measurement, and the shortcomings of the Korotkoff method, extensive investigation has been conducted in the area of continuous, noninvasive blood pressure monitoring and recording. Some of these noninvasive techniques make use of tonometric principles which take advantage of the fact that as blood flows through the arterial vessel, forces are transmitted through the artery wall, through the surrounding arterial tissue and, consequently, are externally available for monitoring. Because the tonometric method of measuring blood pressure is noninvasive, it is used without the risks associated with invasive techniques. Furthermore, in addition to being more accurate than the Korotkoff method discussed above, it has the capability of reproducing the entire blood pressure wave form, as opposed to the limited systolic and diastolic pressure points provided by the Korotkoff method.
A technique for determining intra-arterial blood pressure involves the method of pressing a sensor against the tissue which overlays an artery of interest thereby flattening, or applanating, the underlying artery. This pressing is increased in intensity until a predetermined state of artery applanation is obtained. In this state, certain assumptions can be made regarding the relationship between the forces transmitted to the sensor (through the tissue overlaying the artery) and the intra-arterial blood pressure.
Thus, in view of the above discussion, it can be seen that it is desirable to provide a calibration apparatus which is effective for calibrating the response of the sensor and its associated support system. Such a calibration system should be easy to operate, and ideally, should be usable in the field. Preferably, such a calibration system should also be capable of detecting the temperature dependence of the sensor and thereby, providing the appropriate data, to the blood pressure system which uses the sensor, for compensating for the effects of temperature on the sensor head.
Thus, it is an object of this invention to provide a sensor calibration system which can be used in the field to calibrate a sensor.
It is also an object of this invention to provide a calibration system which does not necessitate dismantling the sensor from a surrounding support structure.
It is still a further object of this invention to provide a sensor calibration system which provides data to compensate for the temperature dependency of the sensor.